The use of conversion coatings for the purpose of inhibiting the corrosion of a metal substrate is well known. Phosphating is a widely used form of metal pretreatment. While phosphate coatings inhibit the corrosion of the metal substrate, the porosity of a phosphate coating results in only a limited measure of protection. For this reason, a phosphatizing process is often followed by a separate rinse process in order to provide for sealing of the pores of the phosphate coating. Chromic acid based solutions have often been used as such sealing rinses for phosphate coatings in the prior art. While chromic acid solutions as sealers are effective, due to the toxicity of the chromic acid solution, waste disposal is difficult. Also, the strongly corrosive nature of a chromic acid solution complicates the storage, handling and use of such solutions.
Sealants for conversion coated metal substrates which avoid the use of chromic acid are known in the art. For example, U.S. Pat. No. 4,517,028 discloses a process employing a substituted phenol polymer for the post treatment of phosphatized or conversion coated metal surfaces. The disclosed process is also described as being effective for treating previously untreated metal surfaces such as aluminum, steel and zinc. The metal surface is treated by contacting with a solution comprising an effective amount of a soluble or dispersible polymer which is a derivative of poly-alkylphenol polymers. U.S. Pat. No. 4,433,015 discloses a composition for the treatment of metal surfaces comprising a dilute solution of a poly-4-vinylphenol derivative or an acid salt of a poly-4-vinylphenol derivative. The treatment enhances the corrosion resistance and paint adhesion characteristics of conversion coated metal surfaces when applied as a post treatment solution. U.S. Pat. No. 4,376,000 discloses a post conversion coating treatment process for metal surfaces which comprises contacting the surface with a dilute solution of poly-4-vinylphenol or the reaction product of aldehyde or ketone and poly-4-vinylphenol.
U.S. Pat. No. 4,220,485 discloses a processes for sealing a phosphatized metal component with a non chromic acid based material which comprises a phosphoric acid, a zinc compound, a heavy metal accelerator and/or crystal refiner and a phosphonate corrosion inhibitor in an aqueous solution. A non-chromate post conversion coating treatment comprising titanium and an adjuvant selected from phosphoric acid, phytic acid, tannin, the salts and esters thereof and hydrogen peroxide is disclosed in U.S. Pat. No. 4,110,129. A post conversion coating treatment containing a melamine-formaldehyde resin and preferrably a vegetable tannin is disclosed in U.S. Pat. No. 4,039,353. A zirconium containing compound is disclosed as a sealer for a conversion coating in U.S. Pat. No. 3,966,502. U.S. Pat. No. 3,493,440 discloses a post phosphate sealer comprising a dilute aqueous solution of a primary phosphate of ammonia, amines and mixtures thereof. U.S. Pat. No. 4,917,737 discloses a method and composition for sealing a phosphate conversion coating comprising contacting a phosphate conversion coating with a substituted iminodimethylene diphosphonic acid.
U.S. Pat. No. 4,514,325 discloses the use of secondary alcohols, derivatives thereof and heterocyclic primary alcohols, like tetrahydrofurfuryl alcohol as a solvent in metal cleaning and degreasing compositions.
The advantages to be gained through the use of sealers to improve the corrosion resistance of conversion coatings has been recognized, as has the objectionable properties of typical chromium compounds employed as a sealer. The most commonly described non-chromium sealers, based upon amines, tannins, aminoalkylated-poly-vinylphenol and heavy metals have not earned wide acceptance. This is principally due to the disappointing performance in retarding corrosion. Furthermore, some of the heavy metal based sealers may pose significant waste disposal problems.